The present invention relates to crosshole seismic testing and data gathering and, more specifically, seismic sources for insertion in boreholes.
Crosshole seismic testing has been commonplace for a considerable time; first in the petroleum industry, and later used in a broader spectrum of geological engineering areas. Crosshole testing incorporates placement of a seismic source in one borehole, and a seismic receiver or geophone in another borehole. Analysis of the seismic data received by the geophone provides valuable information on soil types, soil compaction, deposits, faults, and other information useful in geological information.
Useful data and data analysis requires repeatable seismic sources and an ability to distinguish between shear and compression waves generated by the sources. Compression waves are characterized by wave front oscillation in the direction of wave propagation, while shear waves oscillate transversely to the direction of wave propagation. Earlier seismic sources, such as explosive or spark sources, produce primarily compression waves. An example of such a device is disclosed in U.S. Pat. No. 5,305,287 by the applicant. The shear waves are relatively weak and the phase relationship of successive events is random, limiting the usefulness of cumulative or "stacking" methods of data accumulation.
Other seismic sources utilize oscillating masses to provide shear or compression waves. One such device is disclosed in U.S. Pat. No. 5,137,109. These devices require complex hydraulic, electromagnetic, or pneumatic arrangements and require special equipment to transport and power the devices.
There is a substantial need for a simple, lightweight seismic source which provides accurate, repeatable compression and shear waves for seismic analysis.